1. Field of the Invention
The invention pertains to piezoelectric resonators used to control oscillator frequencies and used in filter circuits. More particularly this invention pertains to resonators which utilize a layer of piezoelectric material sandwiched between two conducting electrodes to provide a mechanical, i.e. acoustic, and an electrical resonance. For use at microwave frequencies, the electrodes typically are fabricated on the piezoelectric layer by desposition techniques.
2. Description of the Prior Art
FIG. 1 depicts a resonator 11 of the prior art consisting of a layer of piezoelectric material 12 having conducting electrodes 13 and 14 located on its top and bottom surfaces respectively. In the prior art the electrodes are usually made of gold or other suitable metal having high electrical conductivity. A voltage applied between the electrodes produces an electric field within the piezoelectric layer, which electric field interacts with the mechanical or acoustic resonances of the device to provide an electrical resonance in response to a sinusoidal voltage applied to the electrodes.
Such piezoelectric resonators exhibit both series and parallel electrical resonances at their terminals, which resonances can be used in the synthesis of bandpass filters in various circuit configurations. The frequency increment between the series and parallel resonances is an important factor in determining the bandwidth that can be exhibited by the filter. In general the greater the frequency increment between the series and parallel resonances, the greater the bandwidth that may be exhibited by the filter. One of the factors that determines the size of the frequency increment is the piezoelectric coupling coefficient, K2e, referred to as "effective K squared" and defined in "High-Q Microwave Acoustic Resonators and Filters" IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, pp. 2139-2146 and in "Development of Miniature Filters for Wireless Applications", IEEE Trans. Microwave Theory and Techniques, Vol. 43, No. 12, December 1993, pp. 2933-2939.
The fabrication of piezoelectric resonators for use at microwave frequencies is well known in the prior art. See the descriptions in the specification of U.S. Pat. No. 5,894,647, and see the references to prior art cited therein. See also "Microwave Acoustic Resonators and Filters," by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139; Guttwein, Ballato and Lukaszek, U.S. Pat. No. 3,694,677; and "Acoustic Bulk Wave Composite Resonators", Applied Physics Letters 38(3) by Lakin and Wang, Feb. 1, 1981.
FIG. 2 depicts a resonator 21 of the prior art consisting of a layer of piezoelectric material 22 and conducting electrodes 23 and 24, all of which are supported on a substrate 25 by intervening layers 26 of different materials. The resonator depicted in FIG. 2 is referred to herein as a solidly mounted resonator ("SMR"). By suitable selection of the materials in the intervening layers, and of the thicknesses of the intervening layers, these intervening layers can be made to present a low or a high acoustic impedance to resonator 21 at the interface 27 between electrode 24 and intervening layers 26. The intervening layers typically are one-quarter wave-length in thickness and alternate having high and low acoustic impedances. The fabrication of such resonators upon such intervening layers is well known in the art. See e.g. U.S. Pat. Nos. 3,414,832 and 5,373,268 and 5,821,833 and see "Solidly Mounted Resonators and Filters", 1995 IEEE Proc. Ultrasonics Symposium, pp. 905-908.
For methods of analysis and further descriptions of reflectors and resonators see Lakin, "Solidly Mounted Resonators and Filters, 1995 IEEE Proc. Ultrasonics Symposium, pp. 905-908 and Lakin et al. "Development of Miniature Filters for Wireless Applications", IEEE Trans. on Microwave Theory and Techniques, Vol. 43, No. 12, December 1996, pp. 2933-2939.
The electrical characteristics of such piezoelectric resonators are also affected by various loss mechanisms, two of which are the electrical losses arising from the electrical currents flowing within the electrodes and the mechanical losses associated with the acoustic waves, i.e. mechanical deformations, within the piezoelectric layer and within the conducting electrodes. For most applications, resonators having lower losses will provide better performance and can be used to obtain filters having wider bandwidths.